In recent years, an integration degree has been considerably improved in semiconductor integrated circuits, and superiority in not only a mechanical precision but also electrical characteristics has been demanded to obtain an integrated circuit having high performance/reliability/production yield. With such a demand, further rigorous conditions are set with respect to a quality of a silicon single crystal substrate (which will be simply referred to as a substrate in some cases) used to fabricate a device, e.g., a semiconductor integrated circuit, and fabricating a silicon single crystal substrate with an improved crystal quality has been demanded.
As a method of obtaining a silicon single crystal from a semiconductor polycrystal material, e.g., silicon, there is a Czochralski method (which will be referred to as a CZ method hereinafter. It is also called a pulling method.) of pulling a seed crystal from a raw material melt to obtain a silicon single crystal. The silicon single, crystal grown by this CZ method is subjected to slicing, lapping, chamfering, polishing, and others, thereby fabricating a silicon single crystal substrate. When this silicon single crystal substrate is subjected to a high-temperature heat treatment to eliminate defects in the vicinity of a substrate surface as much as possible, a quality of a device can be improved. A substrate taking advantage of such characteristics is a substrate having a denuded zone layer (a DZ layer), and its superiority has been substantially proved.
On the other hand, in a bulk portion of a substrate, increasing in gettering performance of capturing an impurity, e.g., a heavy metal by forming bulk micro defects (which will be referred to as BMDs hereinafter) made of, e.g., oxide precipitates with a high density is demanded. For example, when performing a device formation heat treatment with respect to a substrate, it is often the case that the substrate is exposed to contamination of impurities, e.g., heavy metals, the heavy metals adversely affect device operations, and hence it must be removed from the vicinity of a surface that is a device forming region. As a method of meeting this demand, there are gettering techniques.
In various gettering technologies, there is a method called intrinsic gettering (which is also called “internal gettering”).
This method is a method of performing a precipitation heat treatment to form BMDs in a bulk of a silicon single crystal substrate that is not used for device formation, and utilizing these BMDs as getter sites for impurities, e.g., heavy metals. For example, although a silicon single crystal grown by the CZ method unavoidably contains oxygen at a fabrication stage, its oxygen concentration can be controlled, and silicon single crystal substrates having various oxygen concentrations can be fabricated in accordance with purposes. When a heat treatment is applied to such a silicon single crystal substrate, oxygen contained in the crystal is precipitated, and BMDs made of, e.g., oxide precipitates are formed in the substrate. Each circumference of the BMDs includes a distortion of a crystal lattice to no small extent, and this distortion captures an impurity, e.g., a heavy metal.
An oxide precipitate is apt to be formed in the silicon single crystal substrate when an electrical resistivity is low in case of a p-type substrate, and it is hard to be formed when an electrical resistivity is low in case of an n-type substrate. In recent years, a substrate with an ultralow resistance having an electrical resistivity that is less than 10 mΩ·cm (0.01 Ω·cm) is often used, and importance of density control of BMDs formed in the substrate with the ultralow resistance has been increased.
As an evaluation method for BMDs, an infrared light scattering tomograph method of irradiating a substrate with infrared laser beams and observing light scattered by BMDs is known, but a penetration ratio of infrared light is lowered when an electrical resistivity of the substrate is low, and hence a measurement precision is greatly lowered when the electrical resistivity is less than 10 mΩ·cm. Further, when the electrical resistivity of the substrate becomes 5 mΩ·cm or below, BMDs cannot be substantially evaluated based on the infrared light scattering tomograph method.
As a method of chemically detecting crystal defects by preferential etching, a Secco fluid, a Sirtl fluid, and a Wright fluid are well known. However, these preferential etchants contain harmful chrome, and hence a waste liquid process becomes a problem. Thus, a so-called chromeless etchant which does not contain chrome has been developed (Japanese Unexamined Patent Application Publication No. 59-94828, Japanese Unexamined Patent Application Publication No. 07-263429, and Japanese Unexamined Patent Application Publication No. 2004-235350).
However, in case of etchant compositions disclosed in Japanese Unexamined Patent Application Publication No. 59-94828 and Japanese Unexamined Patent Application Publication No. 07-263429, preferentially etching a substrate having an ultralow resistance to detect BMDs is difficult. Furthermore, an etching method disclosed in Japanese Unexamined Patent Application Publication No. 2004-235350 is a crystal defects evaluation method for an SOI substrate, and optimization of preferentially etching a surface of a substrate having an ultralow resistance to elicit BMDs is not performed.
In JIS H0609:1999, solutions A to D containing a hydrofluoric acid, a nitric acid, an acetic acid, and water are specified as preferential etchants containing no chrome. However, they are effective with respect to a silicon single crystal substrate having an electrical resistivity higher than 20 mΩ·cm, but their preferential etching properties are very low with respect to an ultralow-resistance silicon single crystal substrate having an electrical resistivity that is less than 10 mΩ·cm, and they are not suitable for evaluation of BMDs.